You know preaching or evangelizing is not about knowing all the scriptures off by heart, and having ready answers like some door-to-door salesman.
It starts with keeping your own personal testimony to righteous living and faith. Believe me others at school and your friends will notice. You will be tempted; like here smoke this joint or have a drink of Jack Daniel's. Everytime you refuse to join in with sinful behaviour they will laugh at you, and call you a square or worse. But in time you will actually be respected for your stand, your personal integrity and faith. In a world that lacks any of this, people do respect consistency and an honest christian that keeps his testimony intact. This by itself is evangelizing to your friends, school mates, sports team, neighbours and so on.
When people ask you why you are the way you are in faith or in being different and not "naughty" like others; then you will have an answer. Jesus, who is God judges the actions of everybody alive on Earth. As a disciple, a follower of Jesus you neither want to offend Jesus and the Gospel, nor do you want to disqualify yourself from salvation. You simply talk about what Jesus means to you, your prayer life and the blessings and answered prayer you receive.
It is called seed planting, where the Word of God is the seed. People's hearts can be like stony ground, shallow soil or good fertile soil. That part you have no control over. Your job is simply to be a farmer and plant seeds. You just start off with being a light in a dark world. Jesus is the light of the world.
Now you know why my avatar is a lighthouse.
"Ye are the light of the world. A city that is set on a hill cannot be hid... Let your light so shine before men, that they may see your good works, and glorify your Father which is in heaven" Matthew 5: 14-16